The heart is a series of pumps that are carefully controlled by a very special electrical system. This electrical system attempts to regulate the heart rate between 60 and 100 beats per minute. The initial electrical signal originates near the top of the upper chamber on the right side of the heart. This chamber is called the right atrium and the special tissue that generates the signal is called the sino-atrial or SA node.
The electrical signal continues in a downward fashion through the atrio-ventricular or AV node, where the signal is slowed slightly by special tissue. The AV node is the “doorway” or relay station to the bundle of His (pronounced Hiss), and the bundle branches in the lower chambers of the heart.
After passing through the left and right bundle branches, the impulse arrives at the Purkinje fibers, where it is transmitted to the muscle cells of the left and right ventricles. Because of the specialized way in which the impulse is transmitted, the ventricles contract almost simultaneously.
With normal conduction, the cardiac contractions are very organized and timed so that the top chambers (the atria) contract before the lower chambers and the heart rate is maintained between 60 and 100 beats per minute.
Abnormally fast heart rates are called tachycardias. As used herein, the term tachycardia means a heartbeat at a rate which is abnormally high and accordingly considered to be dangerous if permitted to continue, or any arrhythmia involving recognizable heartbeat patterns containing repetitions which are in excess of a periodic heartbeat within a safe range.
When the ventricular chambers beat too quickly, the arrhythmia (i.e., unusual heart rhythm) is known as ventricular tachycardia. When ventricular tachycardia (VT) occurs, the ventricles may not be able to fill with enough blood to supply the body with the oxygen rich blood that it needs. Symptoms of VT include feeling faint, sometimes passing out, dizziness, or a pounding in the chest.
Ventricular tachycardia may be controlled by medication in some cases. If medications are not effective, the physician may elect to control the rhythm by electrical methods. The most common electrical therapy for VT is implantation of a device known as an Implantable Cardioverter Defibrillator (ICD). The ICD applies an electric shock to the heart muscle to interrupt or disrupt the fast rhythm. The electric shock may be in the form of specially timed pacemaker pulses (unfelt by the patient) or by high voltage shock. The high voltage shock, if required, is usually painful to the patient. Accordingly, it is preferential to use pacemaker pulses (also referred to as pacing pulses).
Tachycardias can result due to any number of reasons. For example, patients who have had myocardial infarctions, or other diseases that create scarring in the ventricular region of the heart, often develop monomorphic ventricular tachycardias. A monomorphic ventricular tachycardia (MVT) is a type of tachycardia that originates from one ventricular focus. These tachycardias often arise in and around the area of scarring on the heart. They are typically uniform and typically occur at a regular rate. Faster MVTs are often associated with hemodynamic compromise, whereas slower MVTs can be more stable.
Anti-Tachycardia Pacing (ATP) has been used to convert ventricular tachycardias into normal sinus rhythm. However, conventional ATP has not proved to be one hundred percent successful at returning the heart to normal sinus rhythm. Additionally, in a rare case, conventional ATP will accelerate the rhythm to ventricular fibrillation. Accordingly, improved methods and apparatuses for decreasing the failure rate of ATP are required. Some of the prior patent documents which teach ATP using low voltage shock therapy systems include U.S. Pat. No. 4,408,606; U.S. Pat. No. 4,398,536; U.S. Pat. No. 4,488,553; U.S. Pat. No. 4,488,554; U.S. Pat. No. 4,390,021; U.S. Pat. No. 4,181,133; and U.S. Pat. No. 4,280,502.
Tachycardia is often the result of electrical feedback within the heart; a natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. By interposing a stimulated heartbeat (i.e., a pacing pulse), the feedback loop may be disrupted. For example, patients with MVT can often times be successfully paced out of the tachycardia using a rapid burst of high rate pacing. The burst consists of a selected number of pulses all delivered at the same rate, an accelerating rate, or an alternating accelerating/decelerating rate. The mechanism that determines success of the burst is the ability to peel-back the refractories between the pacing site and the origin of the arrhythmia and penetrate the reentrant loop.
In conventional ATP, anti-tachycardia pacing pulses are delivered using two electrodes within the right ventricle (RV). It has been found that RV only ATP is clinically effective in terminating VT with 90% typical success rates. Clinical studies have indicated that simultaneous BV ATP is comparable to RV only ATP. See Bocchiardo et al., “Efficacy of Biventricular Sensing and Treatment of Ventricular Arrhythmias,” PACE, Vol. 23, November 2000, pp. 1989-1991. In the Bocchiardo study, the BV pacing was accomplished using an RV tip electrode, an RV proximal electrode, and an LV tip electrode. The Bocchiardo study concluded that “[t]he success rates of spontaneous VT termination by BV ATP versus RV ATP were comparable.”